A known mixing apparatus described in the German Pat. No. 738 787, which represents a type of mixer still used extensively in industry, has two rotating shafts on which are disposed two main mixing and conveying vanes each of which extends toward the middle of the shaft from the ends thereof. The pitch of these vanes is approximately 60.degree. and the upstream end face of the vane is flush with the end face of the mixing chamber. Near the downstream end of the main vanes, the shaft carries auxiliary vanes which have the same pitch as the main vanes but the opposite direction of pitch. Generally, the known mixer which normally handles rubber, performs well in transporting the mechanical energy into the substance to be kneaded. However, it has been shown that locally high temperatures may occur due to the reltively poor heat transfer properties of the apparatus and its contents.
Another known internal mixer is described in the British Pat. specification No. 431,012 and includes two intermeshing kneading rollers each of which has symmetric and oppositely disposed surface features which engage one another while the two shafts or rollers rotate at the same speed and in opposite directions. The point of closest contact between the rollers defines a gap in which temperature peaks can occur due to friction. This internal mixer is effective principally between the two rollers but not between the rollers and the housing and the usable volume is relatively small.
Still another internal mixer, is described in the German Pat. No. 1 679 879, with conical roller or shaft cores and non-interacting vanes which provides a generally well-defined and intensive axial flow of material.